Motion blur caused in liquid crystal panels is considered to be caused by the characteristics of the liquid crystal adopting a hold-type system, in which the liquid crystal holds the charge and maintains emission (refer for example to patent document 1).
In conventional cathode ray tubes (CRT) and the like in which electron beams are irradiated to fluorescent bodies, the form of emission of pixels adopts an impulse-type system, in which emission time is short.
The human vision with respect to a moving object can follow the movement smoothly, so that when a hold-type emission is adopted in which the luminance level is held until the next frame, an afterimage is caused via a time integration effect. This phenomenon is called a motion blur.
One method for reducing such motion blur is to double the frame rate to thereby approximate the liquid crystal display to an impulse type display. This technique is called an FRC (frame rate converter) which is already applied to practical use in liquid crystal displays. Examples of this technique for doubling the rate include simple repeating of the same pictures, or interpolating frames between frames via linear interpolation, but unnaturality of movement (jerkiness, judder) occurred, or the motion blur caused by the hold-type emission could not be sufficiently improved, and the picture quality could not be improved sufficiently.
Therefore, in order to eliminate the effect of jerkiness and improve the quality of the motion picture, an FRC technique of a motion-compensating frame interpolating process has been invented and put to practical use.
One example of this circuit is shown in FIG. 3, and the contents of the process will be described below.
The input picture signal B is initially divided in a preprocessing filter 30 into blocks having a predetermined number of pixels and a predetermined number of lines, and filtering is performed within the block to remove unnecessary noise or the like. This block is called a detection block.
Next, in a motion vector detecting part 32, a motion vector (detected vector) of each detection block between the preceding frame and the current frame delayed via the frame memory 31 is computed by selecting candidate vectors from the vector memory 33 and via an iterative gradient method. The details of the contents of the motion vector detecting process (the portion surrounded by dotted line frame 3A) will follow.
An interpolated vector evaluating part 34 evaluates the detected vector obtained in the motion vector detecting part 32, and based on the evaluation result, allocates an interpolated vector for each interpolated block between frames. Here, the interpolated block has a smaller size than the detection block. Actually, the interpolated picture to be generated is divided into interpolated block units, and the detected vector allocated to the detection block is allocated to an interpolated block within an interpolated picture that the vector passes when the detected vector is extended from the preceding frame to the current frame. If a different detected vector has already been allocated, then an evaluation is performed to determine which of the vectors is more appropriate, and the more appropriate vector is adopted.
Next, an interpolated picture is generated in an interpolated frame generating part 36 using the interpolated vector allocated by the interpolated vector evaluating part 34. The example of the procedure will be described with reference to FIG. 4.
A dotted line frame W denotes the detection block to be processed. In order to obtain data of one pixel P within the detection block of the interpolated picture, points P1 and P2 are obtained by extending the interpolated vector allocated to the position of pixel P to the preceding and following original pictures. Then, by interpolating the four pixels around points P1 and P2, the data on the interpolated pixel P can be obtained.
The interpolated picture is obtained by the above process, and a double rate conversion is realized by outputting data corresponding to the original picture, the interpolated picture and the original picture via a time base converting part 37 considering the timing with the original picture.
FIG. 5 shows a detailed configuration of the block related to motion vector detection surrounded by the dotted line frame 3A of FIG. 3, and the contents of the process will be described hereafter.
At first, in a vector selecting part 52, an initial vector of a data of a preprocessed input signal A and a block corresponding to the same position as signal A of a preceding frame delayed via a frame memory 50 is determined via DFD value comparison using a number of candidate vectors read out from a vector memory 51. The vector memory 51 stores initial vector candidates such as the computed vector close to the target block, the vectors detected in the preceding frame and the frame before the preceding frame, and the average value of the vectors of the whole frame. DFD (displaced field difference) refers to the probability of a vector, which is the sum of absolute values of the difference between frames of the respective pixels within the block, the probability becoming higher as the value becomes smaller. Based on the initial vector selected in this manner, a motion vector calculating part 53 computes a detected vector Va′ of the respective block unit using an iterative gradient method.
The iterative gradient method is described in detail in patent document 2, and a simple principle of the method is shown in FIG. 6. A block B1 of the current frame is obtained by moving a block B0 of a current frame which is positioned at the same position as block B of a preceding frame for an initial vector V0. The gradient method is applied to the block B1 and the block B of the preceding frame to obtain a primary vector V1. Then, the gradient method is applied to block B2 of the current frame denoted by V1 and the block B of the preceding frame to obtain a secondary vector V2, and the process goes on in a similar manner, according to which the gradient method is applied to block B3 of the current frame denoted by V2 and the block B of the previous frame. In general, the value of the vector size obtained via the gradient method becomes smaller in the following manner, |V0|>|V1 |>|V2 |>|V3 |, therefore, as the number of application of the method increases, the detected vector precision being calculated is improved. However, in the actual circuit, the number of times of the process is limited, since the process must be performed in real time.
The above description summarized the art of film rate conversion (FRC) put into practical use.
[Patent document 1]
Publication of Japanese patent No. 3295437
[Patent document 2]
Publication of Japanese patent No. 2930675